text: Once this water is inside the boiler or steam generator, the process of adding the latent heat of vaporization begins. The boiler transfers energy to the water by the chemical reaction of burning some type of fuel.  The water enters the boiler through a section in the convection pass called the economizer. From the economizer, it passes to the steam drum, from where it goes down the downcomers to the lower inlet water wall headers. From the inlet headers, the water rises through the waterwalls. Some of it is turned into steam due to the heat being generated by the burners located on the front and rear waterwalls (typically). From the waterwalls, the water/steam mixture enters the steam drum and passes through a series of steam and water separators and then dryers inside the steam drum. The steam separators and dryers remove water droplets from the steam; liquid water carried over into the turbine can produce destructive erosion of the turbine blades. and the cycle through the waterwalls is repeated. This process is known as natural circulation.
  Geothermal plants need no boiler since they use naturally occurring steam sources. Heat exchangers may be used where the geothermal steam is very corrosive or contains excessive suspended solids.  Nuclear plants also boil water to raise steam, either directly passing the working steam through the reactor or else using an intermediate heat exchanger.